The disclosure relates generally to bearing assemblies for mud motors used in drilling of oil, gas, and water wells. More particularly, the disclosure relates to pressure compensation systems for oil-sealed bearing assemblies.
It has become increasingly common and desirable in the oil and gas industry to use “directional drilling” techniques to drill horizontal and other non-vertical wellbores, to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical wellbores. In directional drilling, specialized drill string components and “bottomhole assemblies” (BHAs) are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a wellbore of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “mud motor”) incorporated into the drill string immediately above the drill bit. A typical mud motor includes several primary components, as follows (in order, starting from the top of the motor assembly): a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component); a power section comprising a positive displacement motor of well-known type, with a helically-vaned rotor eccentrically rotatable within a stator section; a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and a bearing section comprising a cylindrical mandrel coaxially and rotatably disposed within a cylindrical housing, with an upper end coupled to the lower end of the drive shaft, and a lower end adapted for connection to a drill bit. The mandrel is rotated by the drive shaft, which rotates in response to the flow of drilling fluid under pressure through the power section. The mandrel rotates relative to the cylindrical housing, which is connected to the drill string.
Directional drilling allows the well to be drilled out at an angle. A bent housing motor is used to form a curved well path. The bent housing is usually located above the bearing section and below the power section. The distance from the bit to the location of the bend in the housing is the bit-to-bend distance. A shorter bit-to-bend distance reduces the bit offset, allowing for a higher build rate for a given bend size. A shorter bit-to-bend distance also reduces the moment arm, reducing the bend stress at the bend. Thus, a shorter bit-to-bend makes the motor easier to orient and control and allows a sharper hole curvature to be made or the same curvature can be achieved with less bend and, subsequently, less overall stress in the motor. It is desirable to minimize the bit-to-bend when drilling non-straight wellbores.
In drilling processes using a mud motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the mud motor to generate power to rotate the drill bit.
The bearing section must permit relative rotation between the mandrel and the housing, while also transferring axial thrust loads between the mandrel and the housing. Axial thrust loads arise in two drilling operational modes: “on-bottom” loading, and “off-bottom” loading. On-bottom loading corresponds to the operational mode during which the drill bit is boring into a subsurface formation under vertical load from the weight of the drill string, which in turn is in compression; in other words, the drill bit is on the bottom of the wellbore. Off-bottom loading corresponds to operational modes during which the drill bit is raised off the bottom of the wellbore and the drill string is in tension (i.e., when the bit is off the bottom of the wellbore and is hanging from the drill string, such as when the drill string is being “tripped” out of the wellbore, or when the wellbore is being reamed in the uphole direction). This condition occurs, for instance, when the drill string is being pulled out of the wellbore, putting the drill string into tension due to the weight of drill string components. Tension loads across the bearing section housing and mandrel are also induced when circulating drilling fluid with the drill bit off bottom, due to the pressure drop across the drill bit and bearing assembly.
Bearings contained in the bearing section of a mud motor may be either oil-lubricated or mud-lubricated. In an oil-sealed bearing assembly, the bearings are located within an oil-filled reservoir in an annular region between the mandrel and the housing, with the reservoir being defined by the inner surfaces of the housing and the outer surface of the mandrel, and by sealing elements at each end of the reservoir. Because of the relative rotation between the mandrel and the housing, these sealing elements must include rotary seals.
Mud motor bearing sections also include multiple radial bearings to maintain coaxial alignment between the mandrel and the bearing housing. In an oil-sealed assembly, the radial bearings can be provided in the form of bushings disposed in an annular space between the inner surface of the housing and the outer surface of the mandrel.
An oil-sealed bearing assembly must incorporate pressure compensation means, whereby the volume of the annular oil reservoir is automatically adjusted to compensate for changes in oil volume due to temperature changes. In addition, certain types of elastomeric rotary seals (such as KALSI SEALS®) are designed to slowly pump oil underneath the seal interface, and this causes a gradual reduction in oil volume which also must be compensated for. For optimum performance of the rotary seal, it is ideal for the sealing surface of the mandrel to be as wear-resistant as possible, while having a very fine surface finish.
A common method of providing pressure compensation in an oil-sealed bearing assembly uses an annularly-configured piston disposed within an annular region (or “piston chamber”) between the housing and mandrel. The outer diameter (OD) of the piston is sealed against the inner bore of the housing (by means of one or more sliding seals, such as O-rings), and also may incorporate anti-rotation seals to ensure that the piston does not rotate relative to the housing. The inner diameter (ID) of the piston is sealed against the mandrel by means of a rotary seal, which rotates relative to the mandrel during operation, and also slides axially along the mandrel as the piston moves. The rotary seal and sliding seals associated with the piston thus define the upper end of the oil reservoir within the bearing assembly.
A sufficient length of the mandrel below the piston's initial position must remain uninterrupted to accommodate the piston travel that will occur as oil volume varies over time (whether due to temperature change or oil loss). The housing bore must be similarly uninterrupted along this length, forming a cylindrical oil reservoir. Therefore, a significant length of the mandrel and housing in a conventional oil-sealed mud motor bearing section is dedicated to the pressure-compensating piston.
For optimum performance of the rotary seal, it is ideal for the sealing surface of the mandrel to be as wear-resistant as possible, with a very fine surface finish. This is typically provided through the use of a surface treatment such as an abrasion-resistant, diamond-ground coating. To accommodate axial translation of the piston within the piston chamber, the surface treatment of the mandrel needs to be provided over a length corresponding to at least the range of travel of the piston's rotary seal, and preferably the full length of the piston chamber.